1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive film-type device. More particularly, the present invention relates to a piezoelectric/electrostrictive film-type device used for an actuator utilizing bending displacement such as a microphone or sensors for sensing fluid properties, sound pressure, microgravity, acceleration, or the like, such as a viscosity sensor.
2. Description of Related Art
Piezoelectric/electrostrictive film-type devices have been conventionally used as an actuator or a sensor. A piezoelectric/electrostrictive film-type device used as a sensor is utilized for measuring fluid properties such as density, concentration, or viscosity, as disclosed in JP-A-8-201265, for example. This type of device is used as a sensor by utilizing the correlation between the amplitude of a piezoelectric vibrator and the viscous resistance of a fluid which comes in contact with the vibrator.
In general, the vibration mode in a mechanical system such the as vibration of a vibrator, may be replaced with an equivalent circuit in an electrical system. A piezoelectric/electrostrictive film-type vibrator is caused to vibrate in a fluid, and the vibrator is subjected to mechanical resistance based on the viscous resistance of the fluid. This allows the detection of a change in the electrical constant of the equivalent circuit of a piezoelectric which makes up the vibrator, whereby fluid properties such as viscosity, density, or concentration can be measured. A measurable fluid refers to liquid and gas, and not only includes a single-component liquid such as water, alcohol, and oil, but also includes a liquid, slurry, and paste in which a medium soluble or insoluble in a liquid such as mentioned above is mixed or suspended therein.
As the detection target electrical constant, a loss factor, phase, resistance, reactance, conductance, susceptance, inductance, capacitance, and the like can be given. In particular, a loss factor or phase having one maximum or minimum change point near the resonance frequency of the equivalent circuit is preferably used. Not only fluid viscosity, but also density or concentration can be measured thereby. For example, the sulfuric acid concentration in a sulfuric acid aqueous solution can be measured. As the index for detecting a change in the vibration mode, a change in the resonance frequency may be used instead of the electrical constant if acceptable from the viewpoint of measurement accuracy and durability.
In this type of piezoelectric/electrostrictive film-type device, as disclosed in JP-A-2002-26 1347, an auxiliary electrode 8 is formed at a position independent of a lower electrode 4 layered on a ceramic substrate 1 including a thin diaphragm portion 3 and a peripheral thick portion 2 as shown in FIG. 1 so that a part of the auxiliary electrode is positioned under a part of a piezoelectric/electrostrictive film 5. This configuration enables an upper electrode 6 to be continuously formed on the auxiliary electrode 8 and the piezoelectric/electrostrictive film 5 without breaking, whereby connection reliability of the upper electrode 6 is improved. In FIG. 1, a measurement target fluid is introduced into a cavity portion 10 through a through-hole 9. Stable device characteristics and a device which can be applied under any use conditions can be obtained by continuously forming the auxiliary electrode 8 from the thin diaphragm portion 3 to the thick portion 3.